Typical of the quality control operations performed in a printing plant is the control of image quality exercised by zonal ink adjustments to produce printed density and printed color within a desired set of standards. Often the standards are established by a proof sheet, and the operator attempts to make press adjustments to produce a printed sheet which matches the proof. As is well known, ink is typically adjusted in zonal increments across the width of the press, the zonal increments being on the order of one or two inches, and the final adjustment to produce an acceptable image across the sheet requires a good deal of skill and experience on the part of the operator.
Printing consoles typically provide a surface for holding a printed sheet, to allow the operator to mentally associate the image on the printed sheet with the control keys which operate the zonal ink adjustment. Often the sheet is manually positioned on the support, and the manual positioning determines the relationship between the image and the control keys. Thus, if the operator incorrectly positions the sheet, or if the image is not correctly positioned on the sheet, there can be a misregister between the keys which control the ink and the actual printed image on the sheet.
Various schemes have been devised for assisting the operator in correlating the printed image on the sheet with the controls which control the printing. Among such systems is that shown in U.S. Pat. No. 4,639,776 which uses a video camera to produce an electronic image of the printed image of a sheet disposed on the console. An image mixer serves to mix the video image of the sheet with internal information relating to ink zones, and to display such mixed information on a separate CRT. The CRT thereby displays the entire printed image and correlates the printed image with the zones for adjustment. However, it does not directly correlate the ink keys on the console with the zone, and thus it requires a measure of operator correlation in order to produce a desired correction to bring the image into conformity with the proof. The problem is made even more complex because the CRT is a relatively small device positioned at the side of the display console, and it requires concentration on the part of the operator to correlate the large printed image with the offset and much smaller displayed image. Furthermore, if the printed sheet is disposed on the table in an incorrect position, the video image displayed on the CRT will not be correctly aligned with the display zones, since the system requires at the outset the physical positioning of the sheet to provide a correlation between the printed image and the control zones.
Systems have also been devised for correlating the position of the sheet with control zones, but they are not entirely satisfactory. For example, German patent document PS 3,232,490 describes a method and apparatus for ascertaining and evaluating ink measuring zones on a printed sheet. The sheet position is taken into account by means for sensing the position of the printed sheet on the console. The sheet position is sensed by scanning elements at the sheet edges, and a computer converts predetermined points for quality control to the new coordinates defined by the altered position of the sheet. As a result, the ink zones can be correlated to the image on the sheet in its actual position on the table, assuming the image is in its expected position on the sheet. However, to the extent the image position varies on the sheet, errors will be introduced. In addition, sensing of image position by such indirect means as sensing of sheet edges results in a reasonably complex system subject to operational problems.